Cognitive and neurophysiological effects of healthy aging and age-related dementias were investigated using standard neuropsychological tests and positron emission tomography to measure brain glucose metabolism and blood flow. Healthy aging was associated with hypometabolism in the frontal lobe, with relative increased activity in parieto-occipital associa on areas. Cerebral blood flow during PET activation by a face matching task correlated with performance accuracy in healthy young and old subjects. The pattern showed higher flow in left fusiform and lingual gyri and right frontal association regions and less flow in bilateral lateral occipitotemporal regions. Studies of visuospatial attention during visual search showed slowing of reaction time with parametric increases in attentional distracters, more so in elderly than young controls. Alzheimer~s disease (AD) patients showed less benefit than controls with decreasing cue size in a visual attention task. Two resting-state metabolic patterns characteristic of AD were identified using regional covariance analysis. These AD patterns differed from that found in patients with frontotemporal dementia. Premorbid intellectual ability was inversely correlated with glucose metabolism in regions of association cortex in AD patients. Early age at onset of dementia was related to greater impairment of visuospatial function. AD patients with a family history of dementia showed greater metabolic deficits in the frontal association regions. An AD subgroup with prominent visual disturbances demonstrated a unique neuropsychological profile with better memory performance than typical patients. These patients showed prominent hypometabolism in parieto-occipital and calcarine cortices. Long-term memory and orientation decline with age among non-demented Down syndrome (DS) adults, whereas language and other functions remain preserved. Basic language skills were inversely correlated with apolipoprotein notype in non-demented DS adults.